1. Field of the Invention
The present invention relates to photopolarimeters, particularly spectrophotopolarimeters. Specifically, the present invention relates to photopolarimeters that use diffraction at more than one grating or multiple diffraction at the same grating to measure all four Stokes parameters of arbitrarily polarized light over a range of wavelengths of light.
2. Description of the Prior Art
The spectral characteristics of grating diffraction are well known and widely exploited in spectroscopy. Light incident on a grating is dispersed into a spectrum, enabling spectroscopy with a slit and grating rotating mechanism, or with a stationary grating and an array detector that intercepts the diffracted light. The spectrum is usually measured using fan-out of only one diffracted order, typically the −1 diffracted order.
The polarization characteristics of grating diffraction are less well known and exploited. In general, each of the orders of diffraction contains different information about the polarization state of the light incident on the grating. By simultaneously measuring the spectra of four or more orders of diffraction, a grating spectrometer can be designed to yield the complete polarization state for the entire spectrum of light.
Azzam U.S. Pat. No. 5,337,146, issued Aug. 9, 1994 and incorporated herein by reference, discloses a division-of-amplitude photopolarimeter (DOAP) based on conical grating diffraction or planar grating diffraction. The Azzam instrument comprises a diffraction grating selected to produce at least four orders of diffraction; a source of an incident light beam whose state of polarization is to be measured, the source oriented such that it directs the light beam to the diffraction grating; at least four photodetectors, each of which intercepts a different diffracted order of the light beam diffracted from the grating; and calculating means responsive to the photodetectors for determining the four Stokes parameters descriptive of the polarization state of the light beam.
If the grating is used in the planar diffraction configuration, the instrument further comprises polarizers that are inserted in the path of at least two of the diffracted beams. One of the diffracted orders of the light may be directed to an alignment detector used to properly align the instrument.
The Azzam patent discloses that instruments that employ planar diffraction have certain advantages over instruments that employ conical diffraction. For example, planar diffraction allows a more compact instrument geometry because all diffracted beams and the detectors can be disposed in one plane.
There are several novel features of the Azzam instrument, generally referred to as a grating-division-of-amplitude photopolarimeter, or G-DOAP, that provide utility in applications. For example, the Azzam G-DOAP delivers a fraction of the entire beam of incident light to each of the detectors used in the instrument. Thus the Azzam instrument differs in principle from a division-of-wavefront photopolarimeter (DOWP) that delivers different portions of the beam of incident light to different detectors. As a result, the Azzam division-of-amplitude instrument is inherently insensitive to transverse spatial variations of the light intensity in the beam that produce errors of measurement in a division-of-wavefront instrument.
The G-DOAP is a complete photopolarimeter, i.e., it determines the signs and values of all four components of the Stokes parameters (conventionally designated S0, S1, S2, S3) that describe the polarization state of the incident light. As such, it provides more information than is obtained by a conventional rotating analyzer photopolarimeter. Also unlike a rotating analyzer device, the Azzam instrument distinguishes right and left circular or elliptically polarized light, distinguishes circular polarized states of light from depolarized light, and measures the sign of the third Stokes parameter, S3.
A further novel aspect of the Azzam instrument is that, since there are no moving parts, all intensity measurements can be obtained simultaneously. Thus the measurement rates are not limited by the time required to perform mechanical adjustments.
An example described in the Azzam patent applied the instrument to measure the polarization state of light at a single wavelength equal to 633 nm. Intensity measurements obtained for zero, −1, −2, and −3 orders of diffraction were used for polarimetric analysis and the −4 order of diffraction was used for alignment.
Unlike this example in Azzam, use of the present invention as a spectroscopic polarimeter for measurements over a range of wavelengths requires that all intensity measurements used for polarimetric analysis be obtained on non-zero orders of diffraction.
To provide a spectroscopic polarimeter based on the Azzam invention, which is the object of the present invention, it is desired that the instrument have a wide spectral bandwidth, high sensitivity for operation with relatively small light intensities, simplicity of construction, and the ability to be readily modified to operate over different spectral ranges that are of interest in applications.
In designing such an instrument, there are several practical issues that must be addressed. A first practical issue in the design of a spectroscopic polarimeter is that light of different wavelengths that is diffracted in different orders will tend to be focused to the same position on the detectors of the instrument, since the angles of diffraction will be equal if the products of wavelength and order number are equal. Four orders of diffraction must be measured. Thus, for example, a polarimeter that applies measurements in orders −1, −2, −3, and −4 will be limited to wavelengths no more than 1.2 times the minimum wavelength of light received due to interference of light diffracted in the −5th order with the −4th order spectrum.
To overcome this problem, spectral blocking filters may be placed to intercept the path of diffracted beams and expand the wavelength range at which the diffraction intensities can be detected without interference from higher-order spectra. It is also possible, but not necessarily desirable in view of other design issues, to adjust the angle of incidence of the light beam on the grating so that both positive and negative diffraction orders can be observed, allowing measurements to be obtained without order overlap and in an increased spectral range, for example, in diffraction orders +2, +1, −1, and −2.
A second practical issue is that the sensitivity of the instrument is limited by the tendency of the diffraction efficiency to be smaller for higher-order diffraction. Thus, instruments designed to detect higher-order diffraction spectra will tend to exhibit smaller sensitivity and require more intense light sources.
A third practical issue is that, as disclosed by Azzam, in the planar diffraction configuration, polarizers must be inserted in the path of at least two of the diffracted beams. Otherwise, the instrument cannot function as a complete polarimeter. While it is not necessary to insert polarizers in the paths of diffracted beams for the conical diffraction instrument, the sensitivity of both the planar and conical diffraction instruments to the polarization state of the light can be increased by inserting such polarizers into the paths of the diffracted beams. In practice, inexpensive sheet polarizers may be used for this purpose, but the sheet polarizers are effective over a limited wavelength range and may not provide sufficient sensitivity of the instrument to the polarization state of light over the full spectral range for which measurements are possible.
A fourth practical issue is that inexpensive polarizers and spectral blocking filters may not be available in the wavelength range of interest. This can occur in the vacuum ultraviolet spectral range, where few materials are available that transmit light.
Thus a primary object of the present invention is to provide a spectroscopic photopolarimeter that applies diffraction at multiple gratings or multiple diffraction events at a single grating.
Another object of the invention is to provide an instrument that can use a relatively large oblique angle of incidence at the grating(s) to increase the sensitivity of the instrument to the polarization state of light.
Another object of the invention is to provide a photopolarimeter that achieves a relatively equal range of light intensities at each of the array detectors at which intensities are measured.
Still another object of the present invention is to provide a photopolarimeter with multiple gratings that are oriented to achieve increased sensitivity to the polarization state of light and allow measurement of the complete state of polarization of light without any polarizers inserted into the paths of diffracted beams.